Compensating for non-linear effects in signal processing with active devices

Abstract

In signal processing circuitry using active devices, non-linearity of the device causes distortion of pure tones and the generation of intermodulation products for more complex signals. The transfer characteristics of such active devices may be considered as the accumulated characteristics of input and output filter stages separated by a non-linear region. To compensate for non-linearities and reduce intermodulation products, a drive circuit comprises pre- and post-correction filters separated by a compensating amplifier. The frequency response of the pre-correction filter corresponds to that of the input filter stage but is reversed in the frequency spectrum; with conventional I.F. the transfer characteristic of the amplifier and frequency response characteristic of the post-correction filter are the respective complements, of those of the non-linear region and pre-correction filter. The circuitry may be extended to compensate for active devices having more than one non-linear region and has particular application to klystron amplifiers used in broadcast television transmitters.

We claim: 
     
       1. A method of compensating for non-linear characteristics of an active device (10), the active device having a transfer characteristic corresponding to the accumulated characteristics of a first filter stage (12) and a first non-linear device (14) in series connection, the method comprising modifying a signal prior to supply to the active device by the steps of: a) filtering the signal by a first pre-correction filter (26) having a frequency response characteristic determined by that of the said first filter stage (12);   b) amplifying the pre-correction filtered signal using a first non-linear amplifier (20) having an input/output transfer characteristic complementary to that of the said first non-linear device (14); and   c) filtering the amplified signal by a first post-correction filter (28) having a frequency response characteristic complementary to that applied at step a).   
     
     
       2. A method according to claim 1, in which said step of filtering by said first pre-correction filter (26) is achieved with a frequency response characteristic substantially identical to that of said first filter stage (12) but is reversed in the I.F. frequency spectrum. 
     
     
       3. A method according to claim 1, in which the active device transfer characteristic corresponds to the accumulated characteristics of n filter stage (12) and non-linear device (14) pairs in series connection, where n is an integer greater than 1, and steps a)-c) are sequentially performed n times for respective ones of the n filter stage and non-linear device pairs. 
     
     
       4. A method according to claim 3, in which the signal modification by steps a)-c) is respectively performed in the order of the nth to first filter stage and non-linear device pair. 
     
     
       5. A method according to claim 1, in which the active device transfer characteristic corresponds to the accumulated characteristics of n series connected filter stages (12) and the said first non-linear device (14), where n is an integer greater than 1, comprising performing step a) in respect of each of the n filter stages in sequence and using respective ones of n pro-correction filters (26), performing step b), and performing step c) sequentially using n post-correction filters (28). 
     
     
       6. A method according to any of claims 1 to 5, comprising the further step of applying differential gain and phase correction (25) to the signal prior to performing steps a)-c). 
     
     
       7. A signal processing circuit comprising a first drive circuit (18) and an active device (10) connected in series between a processing circuit signal input and signal output, the active device (10) having a transfer characteristic corresponding to the accumulated characteristics of a first filter stage (12) and a first non-linear device (14) in series connection, and the first drive circuit (18) comprising, in series connection from a drive circuit signal input: i) a first pro-correction filter (26) having a frequency response characteristic determined by that of the said first filter stage (12);   ii) a first non-linear amplifier (20) having an input/output transfer characteristic complementary to that of the said first non-linear device (14); and   iii) a first post-correction filter (28) having a frequency response characteristic complementary to that of the said first pro-correction filter (26); wherein the output of the said first post-correction filter provides a drive circuit signal output.     
     
     
       8. A circuit according to claim 7, in which the first pro-correction filter (26) frequency response characteristic is substantially identical to that of the said first filter stage (12) but is reversed in the I.F. frequency spectrum. 
     
     
       9. A circuit according to claim 7, in which the active device transfer characteristic corresponds to the accumulated characteristics of n filter stage and non-linear device pairs in series connection, where n is an integer greater than 1, comprising n-1 further drive circuits (30) connected in series with the first drive circuit (18), the m th  drive circuit, where n≧m>1, comprising: i) an m th  pre-correction filter (32) having a frequency response characteristic determined by that of the m th  filter stage;   ii) an m th  non-linear amplifier (36) having an input/output transfer characteristic complementary to that of the m th  non-linear device; and   iii) an m th  post-correction filter (34) having a frequency response characteristic complementary to that of the m th  pre-correction filter.   
     
     
       10. A circuit according to claim 9, in which the n drive circuits (18, 30) are arranged in the order of n th  to 1st and the n filter stage and non-linear device pairs are arranged in the order of 1st to n th , in series from the processing circuit signal input to the signal output. 
     
     
       11. A circuit according to claim 7, in which the active device transfer characteristic corresponds to the accumulated characteristics of n series connected filter stages (12) and the said first non-linear device (14), where n is an integer greater than 1, the first drive circuit further comprising n-1 further pre-correction filters in series connection between the drive circuit signal input and the first pre-correction filter (26), and n-1 further post-correction filters in series connection between the first post-correction filter (28) and the drive circuit signal output. 
     
     
       12. A circuit according to any of claims 7 to 11, further comprising differential gain and phase correction means (25) at the said processing circuit signal input. 
     
     
       13. A circuit according to claim 7 or claim 8, in which the active device is a multiple-cavity klystron, the said first filter stage (12) frequency response characteristic is that of the first to penultimate cavities, and the first non-linear device (14) transfer characteristic is that of the interaction area between the penultimate and final cavities. 
     
     
       14. A circuit according to claim 13, in which the active device is a four cavity klystron amplifier. 
     
     
       15. In a broadcast television transmitter having a klystron amplifier, said klystron amplifier comprising a drive circuit (18) and a klystron (10) driven by said drive circuit, said klystron (10) having a transfer characteristic corresponding to the accumulated characteristics of a first filter stage (12) and a first non-linear device (14) in series connection, and said drive circuit (18) having an input and an output and comprising a non-linear amplifier (20) having an input/output transfer characteristic complementary to that of said klystron first non-linear device (14), the improvement residing in that said drive circuit (18) further comprises a pre-correction filter (26) connected between said drive circuit input and said non-linear amplifier (20), and a post-correction filter (28) connected between said non-linear amplifier (20) and said drive circuit output, wherein said pre-correction filter (26) has a frequency response characteristic determined by that of said first filter stage (12) and said post-correction filter (28) has a frequency response characteristic complementary to that of said pre-correction filer (26). .Iadd. 
     
     
       16.  A method of compensating for non-linear characteristics of an active device (10), the active device having a transfer characteristic corresponding to the accumulated characteristics of a first filter stage (12) and a first non-linear device (14) in series connection, the method comprising modifying a signal prior to supply to the active device by the steps of: a) amplifying the pre-correction filtered signal using a first non-linear amplifier (20) having an input/output transfer characteristic complementary to that of the said first non-linear device (14), and   b) filtering the amplified signal by a first post-correction filter (28) having a frequency response characteristic determined by that of the said first filter stage (12)..Iaddend..Iadd.17. A method according to claim 16, in which the active device transfer characteristic corresponds to the accumulated characteristics of n filter stage (12) and non-linear device (14) pairs in series connection, where n is an integer greater than 1, and steps a) and b) are sequentially performed n times for respective ones of the n filter stare and non-linear device pairs..Iaddend..Iadd.18. A method according to claim 17, in which the signal modification by steps a) and b) is respectively performed in the order of the nth to first filter stare   
     
     
        and non-linear device pair..Iaddend..Iadd.19.  A signal processing circuit comprising a first drive circuit (18) and an active device (10) connected in series between a processing circuit signal input and signal output, the active device (10) having a transfer characteristic corresponding to the accumulated characteristics of a first filter stage (12) and a first non-linear device (14) in series connection, and the first drive circuit (18) comprising, in series connection from a drive circuit signal input: i) a first non-linear amplifier (20) having an input/output transfer characteristic complementary to that of said first non-linear device (14); and   ii) a first post-correction filter (28) having a frequency response characteristic determined by that of said first filter stage (12); wherein the output of said first post-correction filter provides a drive circuit signal output..Iaddend..Iadd.20. A circuit according to claim 19, in which the active device transfer characteristic corresponds to the accumulated characteristics of n filter stage and non-linear device pairs in series connection, where n is an integer greater than 1, comprising n-1 further drive circuits (30) connected in series with the first drive circuit (18), the m th  drive circuit, where n≧m>1, comprising:     i) an m th  non-linear amplifier (36) having an input/output transfer characteristic complementary to that of the m th  non-linear device; and   ii) an m th  post-correction filter (34) having a frequency response characteristic determined by that of the m th  filter   
     
     
        stage..Iaddend..Iadd.21.  A circuit according to claim 20, in which the n drive circuits (18, 30) are arranged in the order of n th  to 1st and the n filter stage and non-linear device pairs are arranged in the order of 1st to n th , in series from the processing circuit signal input to the signal output..Iaddend..Iadd.22. A circuit according to claim 19, in which the active device is a multiple-cavity klystron, the said first filter stage (12) frequency response characteristic is that of the first to penultimate cavities, and the first non-linear device (14) transfer characteristic is that of the interaction area between the penultimate and final cavities..Iaddend..Iadd.23. A circuit according to claim 22, in which the active device is a four cavity klystron amplifier..Iaddend..Iadd.24. In a broadcast television transmitter having a klystron amplifier, said klystron amplifier comprising a drive circuit (18) and a klystron (10) driven by said drive circuit, said klystron (10) having a transfer characteristic corresponding to the accumulated characteristics of a first filter stage (12) and a first non-linear device (14) in series connection, and said drive circuit (18) having an input and an output and comprising a non-linear amplifier (20) having an input/output transfer characteristic complementary to that of said klystron first non-linear device (14), the improvement residing in that said drive circuit (18) further comprises a post-correction filter (28) connected between said non-linear amplifier (20) and said drive circuit output, wherein said post-correction filter (28) has a frequency response characteristic determined by that of said first filter stage..Iaddend.